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Effect of X irradiation on optical properties of Teflon-AF
Radial. Phys. Chem. VoL 41, No. 3, pp. 481—486, 1993
0146.5724/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
EFFECT OF X IRRADIATION ON OPTICAL PROPERTIES OF TEFLON-AF 2 M. S. J.~ii~N,’D. R. Eiu.init’ and D. W. COOKE tDepartment of Physics, Memphis State University, Memphis TN 38152, U.S.A. and 2Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. (Received 9 June 1992; in revised form 14 August 1992) Abstract—Radiation effects in optical-grade amorphous fluoropolymer, Teflon-AF, is investigated by
UV—visible absorption and electron spin resonance (ESR) measurements. When irradiated with lowenergy (40 kVp) X-rays at room temperaturein air, Teflon-AF is found to develop a broad, structureless UV-absorption band in the wavelength interval 200—350 nm. While the UV absorption increases as a function of X-ray dose, with relative rates of approx 2 x l0~Gy~(I x l0-~Gy ‘)in Teflon-AF 1600 (l’eflon-AF 2400), its optical transparency for a given dose of 67.5 kGy, however, remains unaffected. Additional measurements conducted using electron spin resonance (ESR) technique reveal that the observed UV absorption is caused by the X-ray induced peroxy radical (P00). The results also suggest that the inclusion of dioxole monomer in the PTFE chain not only improves the optical clarity of Teflon-AF, as reported, but also increases its radiation tolerance. During a post-irradiation storage in air at RT for about 30 days the peroxy radical is observed to decay, with a concomitant decrease in UV absorption. A tentative model is proposed to explain the radiation damage and recovery mechanisms.
INTRODUC’IlON A new family of amorphous teflon (amorphous fluoropolymer) known as Teflon-AF (developed by Du Pont) is reported (Lowry et al., 1990) to have very high optical transmission property. In the visible—NIR regime the transmission is 95% or better, and in the UV (200—400 nm) it is about 80%. With this high optical transparency, in conjunction with its low surface energy, refractive index between 1.29 and 1.31, and resistance to severe environment (water absorption <0.01%), Teflon-AF shows a great potential for application in optics. For example, optical fibers, cladding for fiber bundles, lenses and a host of other optical components can be made out of this material. It can also be used for optical coatings. While preliminary studies (Lowry et a!., 1990) provide information about the mechanical and thermal stabilities, and the effects of y irradiation on the mechanical properties (elongation at break decreases by 12% and tensile t0Co) at break by 3 kPsi of Teflon-AF, for data no a dose areofavailable, 1 x l0~Gy to our of knowledge, on the radiation effects on its optical properties. This paper reports changes in optical absorption of Teflon-AF due to X irradiation at room temperature (RT) in air. The motivation for this study stems from our search for radiation tolerant fibers and cladding materials to be used in ionizing-radiation environments (detector systems in SSC, for example). Fibers and optical components will also be used in many environments where they may encounter radiation 481
doses sufficient to alter their transparency. Possible application sites are spacecraft, nuclear reactors, natural radioactive deposits, radioactive waste storage areas and the vicinity of nuclear weapons detonations. Studies on commercially available fibers and cladding materials show an appreciable degradation of optical transmission due to radiation damage. The growth and recovery of radiation-induced attenuations in 5i0 2, Ge-Si02 and Ge-P—Si02 were investigated by Friebele ci a!. (1984), who attributed this effect to the formation of color centers due to intrinsic and extrinsic defects. In recent studies, Jahan et a!. (199lb) observed formation of absorption bands due to y irradiation in acrylic light pipes and Wick ci a!. (1991) observed radiation damage and recovery in polymethyl-methyacrylate (PMMA)based light guides due to formation and decay, respectively, of free radicals. In this study free-radical growth and the concomitant change in UV—visible absorption were measured as a function of X-ray dose. Similar measurements were performed a function of storage (in air at RT)also time followingasa maximum dose of 67.5 kGy to monitor recovery of the materials. EXPERIMENTAL
Materials This study concentrated on two new generation teflon materials, Teflon-AF 1600 and Teflon-AF 2400. The Teflon-AF family is made of dioxole monomer and polytetrafluoroethylene (PTFE)
482
M. S.
JAHAN
(Lowry et a!., 1990). Shown below is its chemical structure. F
F
F
F
CF(
“èp3
~
2.5
et a!.
The difference between Teflon-AF 1600 and Teflon-AF 2400 is only in the relative amount of the dioxole monomer in the basic polymer chain. The numbers 1600 and 2400 refer to the glass transition temperatures of 160 and 240°C,respectively. Two pre-polished samples were provided by the Du Pont Corporation. Each of the samples was a compression moulded disk, approx 25 mm in diameter and 3.2 mm thick. Small pieces suitable for ESR and optical measurements were cut from these
(A) Teflon-AF 1600
2.0 ~ 2.4
1.5 Id
Q
a
Id
C
1.0
~0.8 0
I
310
.~
420
I
530
I
640
I
750
Wavelength (nm) 200
0
200
240
280
320
360
400
Wavelength (nm) 4.5
(B)
3.6 Teflon-AF 2400 2.7 I-
V
-
a I-
1.8
d
o
F.,
e
C
0.9
a
b
0 200
240
I 280
I 320
360
400
Wavelength (nm) Fig. I. UV-absorption spectra of X irradiated Teflon-AF 1600 (A) and Teflon-AF 2400 (8). Cumulative X-ray doses are 0.0 (a), 7.5 (b), 15.0 (c), 22.5 (d), 45.0 (e) and 67.5 kGy (f). The inset shows the UV—visible absorbance of the Teflon-AF 1600 following 67.5 kGy of X-ray dose.
Effect of X irradiation on optical properties of Teflon-AF
483
(A) ESR Teflon-AF 1600
25 Gauss
e
(B) Teflon-AF 2400
,
a
___
~
__.) -
—
b
,~
—
-.
C
d
25 Gauss e
Fig. 2. ESR spectra of X irradiated Teflon-AF 1600 (A) and Teflon-AF 2400 (B). Cumulative X-ray exposures are 0.0 (a), 7.5 (b), 15.0 (c), 22.5 (d), 45.0 (e) and 67.5 kGy (f). g~and g 1 are the parallel and perpendicular components, respectively, of the electronic splitting factor.
—
484
M. S.
JAHAN ci a!.
25-mm samples and were used without additional
ci a!., 1992) and hydrogen peroxy (HO
polishing.
ci a!., 1991) radicals, and in superoxide (O~peroxy) defect in topaz (A12F2(Si04)) (Priest ci a!., 1991).
Methods Samples were X-irradiated at room temperature in air employing an X-ray machine with a copper target and operating at 40 kVp and 20 mA. Dose rate at the sample site was 1.5 kGy/min. Maximum dose given to a sample was 67.5 kGy. UV—visible absorbance measurements were conducted using a UV—visible—NIR spectrophotometer (CARY 2390) at wavelengths ranging from 200 to 750 nm. Absorption spectra of a single sample were recorded following a cumulative dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. For ESR measurements a X-band spectrometer, whose cavity operates in TE102 mode with 50 kHz modulation frequency of the magnetic field, was employed. The spectrometer calibration and sensitivity tests, and the determination of free radical concentration in a sample were accomplished by using a ruby standard (NIST). A low microwave power was maintained in the cavity so as to avoid saturation of the ESR signals. Like absorbance measurements, ESR measurements were also conducted on a single sample following each cumulative X-ray dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. The total time a sample was exposed to air between two successive irradiations was, including the time for a measurement (ESR or optical absorption), approx 10 mm. Following a dose of 67.5 kGy the samples were stored in air at RT. Their recovery was then monitored by recording absorbance and ESR data at a regular interval of time ranging from 0 to 30 days.
From radiation damage study of PMMA-based light guides and wavelength shifters, and from a review of similar works by others, Wick ci a!. (1991) noted that the peroxy radicals absorb in the UV but not in the visible. An excellent illustration of this phenomenon is found in the present work. The ESR spectra, shown in Fig. 2, bear the characteristic features of a peroxy radical with anisotropic electronic splitting factors g11 = 2.0399 and g1 = 2.0175 (AH = 46G). A correlation between the peroxy radical concentration and the UV absorption is shown in Fig. 3. It is found that for a given X-ray dose the relative concentration (N16/N24)~,,of peroxy radicals (N16 = concentration of peroxy radicals in Teflon-AF 1600 and N24 = concentration of peroxy radicals in Teflon-AF 2400) is found to be 2.5. The relative absorbance (A16/A24)225 nm, (A16 = absorbance in Teflon-AF 1600 and A24 = absorbance in Teflon-Af 2400, both measured at 225 nm) obtained from the UV-absorption data (Fig. 3) is, however, found to be 2. This difference might be attributable to the methods used to compute them. Recall that radical concentration is obtained from a double integration of the entire ESR spectrum, while the relative absorbance (A16/A24)225 nm is computed at a particular wavelength (225 nm) of the broad UV band. Within experimental uncertainties, therefore, they can be considered approximately equal, (N16/N24) (A16/A24)225 nm. From this correlation one can condude that for a given X-ray dose, about twice as many peroxy radicals are formed in Teflon-AF 1600 than in Teflon-AF 2400, and that the peroxy radicals
RESULTS AND DISCUSSION
thus formed cause UV absorption. Peroxy radicals
Shown in Fig. 1 (A and B) are the absorption spectra of Teflon-AF 1600 and Teflon-AF 2400 recorded following cumulative X-ray dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. Because there is no observable change in the absorbance due to X irradiation at wavelength 2 > 350 nm [see inset, Fig. 1(A)], only the UV (200—400 nm) spectral region is shown in the figures. The concomitant ESR spectra of the samples are shown in Fig. 2 (A and B). The area under an ESR absorption curve, which measures the relative concentration of free radical content in a sample, was obtained by integrating twice the recorded first derivative spectrum and plotting it as a function of X-ray dose (see Fig. 3). Also plotted in Fig. 3 are the absorbance (at an arbitrarily chosen 2 = 225 nm) as a function of X-ray dose It is clear from Fig. 1 (A and B) that Teflon-AF 1600 and Teflon-AF 2400, which differ only in relative absorbance, show similar UV absorption behavior. Their absorption bands spread continuously from 200 to about 350 tim. Similar UV absorptions were observed in methylperoxy (CH302) (Wallington
2) (Crowley
1.5
-
1 0
—
(a)
-
Teflon-AF ~
—
2
-
i
—
0 1.0 ~
•
i o.~ — ~ ~ .~.
0.8
—
0.6
—
,‘
I
I
I
(b)
-
Teflon-AF 2400
—
0.4 02 .
0
-
0.6 0.4
—
0.2
—
—
_•~4 20
40
60
80
X-ray dose (kGy) Fig. 3. Increase in UV-absorbance and integrated ESR intensity are plotted as a function of X-ray dose. (a) Teflon-AF 1600 and (b) Teflon-AF 2400.
Effect of X irradiation on optical properties of Teflon-AF
have been investigated in a large number of polymers, for example, polypropylene (PP) (Becker ci a!., 1988; Carlsson ci a!., 1985, 1980), polytetrafluoroethylene (PTFE) (Ranby and Rabek, 1975) and polyethylene (PE) (Igarashi, 1983; Jahan et a!., 1991a). The mechanisms of formation of peroxy radicals in Teflon-AF can be outlined as follows. Radiation-induced polymer free radicals (P) react with oxygen molecules (02), trapped in or diffused (from outside) into the matrix, to produce peroxy radicals. Xra
~ P
0~
P00
Initiation
(1)
Propagation
(2)
Recall that X irradiation of Teflon-AF and the subsequent ESR and UV—visible absorption measurements were conducted in air, thus making oxygen easily available to previously formed polymer radicals. Furthermore, oxygen, because of its high diffusional mobility and high reactivity with radicals, traps radicals efficiently to produce peroxy radicals, as shown by reaction (2), and promote oxidative reactions. This is consistent with the numerous reports (Sirois and Wigmans, 1985; Gillen and Clough, 1985; Moore and Choi, 1991) on radiationinduced free radicals in polymers and their reactions
1.0
485
(a)
-
Teflon-AF 1600
~
0.8 0.6
-\
0.4
-
0.2
-
1.0
-
UV-absorbance
ESR
~..
0.8 ~ 0.6
~
0.4 0 2
I b
I
\
I
I
‘~•i
I
Teflon-AF 2400
-\ ~
UV-absorbance
-
ESR 0
!—4---—-4-~_~.J_~
~
I
5
30
35
10
15
20
25
Storage time (day) Fig. 4. Fractional changes in UV-absorbance and integrated ESR intensity as a function ofpost-irradiation storage time in air at RT. (a) Teflon-AF 1600 and (b) Teflon-AF 2400.
the radicals in these two samples can be due to initial radical concentration difference [(N 16/N24)e,,= 2.5]. with 02 during and after irradiation (for a review, Based on our computation of the relative concensee Clough, 1988). tration of peroxy radicals [(Nj6/N24)~~r = 2.5] and the A peroxy radical can react with another neighbor- fact that the concentration of dioxole monomer in ing peroxy radical to form a neutral species and/or Teflon-AF 1600 is less than that in Teflon-AF 2400, it can combine with a hydrogen atom by pulling it (Lowry et a!., 1990) we propose TFE (tetrafluorofrom environmental H20 (note, there is no hydrogen ethylene) in the main copolymer chain as the most in the molecular structure of Teflon-AF) and form a probable site for formation of the peroxy radical hydroperoxide. This is shown by reactions (3) and (4). (P0(Y). Similar peroxy radicals were observed in 2P0(Y —* P0 P +0 (3’~ PTFE by Moriuchi et a!. (1970), in PE (polyethylene) 2 2 / by Igarashi (1983), and in ultra-high molecular P00
~
P0~H
(4)
A post-Irradiation storage of Teflon-AF in air ~ therefore, lead to a simultaneous decrease of the ESR and UV-absorption intensities of the peroxy radicals as shown in Fig. 4. It is apparent from the figure that, in the given condition (RT in air), the peroxy radicals decay at a faster rate for the first 5 days, then follow a slow-decay pathway. Without additional data the decay mechanisms cannot be elucidated; however, one can speculate that, initially, either both reactions (3) and (4) can take part in the decay process or that the hydrogen reaches the near-surface radicals faster by reaction (4) leading to a fast decay of the P00~radicals. Following this, P00 radicals will be sparsely populated in the matrix and subsequent decay will be dominated by reaction (4). In this reaction hydrogen (H20 molecules) must travel a long distance to react with P00 radicals, leading to a slow decay process. Using ESR data we find the relative magnitude of the fast (kf) and slow (k,) decay rates, (k~/k~)1640 and (kf/k,)24 20, in Teflon-AF 1600 and Teflon-AF 2400, respectively. This difference (a factor of 2) in the decay rates of
weight PE by Jahan ci a!. (1991a). Measurements are in progress to ascertain whether P00~is a chain or -~ d~ an enu YP~ra ica. CONCLUSIONS
The optical transparency of optical-fiber grade amorphous teflon, Teflon-AF, is found to be unaffected within a given dose of low energy (40 kVp) X irradiation. This material, however, develops a UV-absorption band in the wavelength interval 200—350 nm when it is X-irradiated at RT in air. This UV band is broad and structureless and shows a continuous decrease in absorbance as wavelength increases from 200 to 350 nm. By comparing this result with previously published works, and by correlating it with ESR data, we conclude that the observed UV-absorption is caused by the X-ray induced peroxy radical (P0(Y). Furthermore, by comparing ESR results of Teflon-AF 1600 (containing more PTFE than dioxole monomer in the copolymer chain) with those of Teflon-AF 2400 (containing less PTFE than dioxole monomer in the copolymer
486
M. S.
JAHAN
chain) we conclude that the peroxy radical forms in the TFE segment of the copolymer chain. it is not clear, however, whether this radical is a chain or end (scission) type. Measurements are in progress to clarify this assignment. Without additional measurements post-irradiation decay mechanisms of the peroxy radicals, with concomitant decrease in the UV absorption, cannot be elucidated; however, we present a tentative model that explains the experimental results. Initially, when radical concentration is high, a peroxy radical cornbines with another neighboring peroxy radical and/or a hydrogen pulled from the environmental H 20. This reaction gives rise to a fast decay process, yielding sparsely populated radicals in the polymer matrix. Subsequent decay will be slow because of the reduction in the radical—radical recornbination and, the long distance required for hydrogen to travel before it can find a peroxy radical with which to combine, Acknowledgements—This research is supported in part by the Texas National Research Laboratory Commission through SAHEP Initiative. We are thankful to Du Pont and to R. L. Clough of Sandia National Laboratory for providing the materials used in this study.
REFERENCES
Becker R. F., Carlsson D. J., Cooke J. M. and Chemla S. (1988) Stabilization of polypropylene to y-initiated Oxidation. Polym. Deg. Stab. 22, 313. Carlsson D. J., Dobbin C. J. B. and Wiles D. M. (1985)
ci a!.
absorption spectrum measured by molecular modulation, U.V./diode-array spectroscopy J. Photochem. Photobiol. A: Chem. 60, 1. Friebele E. J., Askins C. G., Gingerich M. E. and Long K. J. (1984) Optical fiber waveguides in radiation environments, II. Nucl. Instr. Mciii. Phys. Res. 81, 355.
Gillen K. T. and Clough R. L. (1985) A kinetic model for predicting oxidative degradation rates in combined radiation—thermal environments. J. Polym. Sci.:Polym. Chem. Ed. 23, 2683. Igarashi M. (1983) Free radical identification by ESR in polyethylene and nylon. 1. Polym. Sci.: Polym. Chem. Ed. 21, 2405. Jahan M. S., Wang C., Schwartz G. and Davidson J. A. (199la) Combined chemical and mechanical effects on free radicals in UHMWPE joints during implantation. J. Biomed. Mater. Res. 25, 1005. Jahan M. S., Ermer D. R., Cox C. B., Cooke D. W. and Bennett B. L. (l991b) Recovery of radiation damage of plastic scintillation and light pipe materials. Bull. Am. Phys. Soc. 36, 2749.
Lowry J. H., Mendlowiz J. S. and Subramanian N. S. (1990) Optical characteristics of the TEFLON AF fluoro-plastic materials. 1330, J.142. Moore J. A. SPIE and Choi 0. (1991) Radiation Effects on Polymers (Edited by Clough R. L. and Shalaby S. W.), ACS Symposium Series 475, Chap. 11, p. 156, American Chemical Society, Washington, D. C. Moriuchi S., Nakamura M., Shimada S., Kashiwabara H. and Shoma J. (1970) ESR spectra of peroxy radicals in y-irradiated polytetrafluoroethylene (PTFE). Polym. 11, 630. Priest V., Cowan D. L., Yasar H. and Ross F. K. (1991) ESR, optical absorption, and luminescence studies of the peroxy-radical defect in topaz. Phys. Rev. B44, 9877. Ranby B. and Rabek 3. F. (1975) Photodegradation, Photooxidation and Photostabilization of Polymers, Principles and Applications, pp.60—71. Wiley, New York.
Direct observations of macroperoxyl radical propagation
Sirois Y. and Wigmans R. (1985) Effects of long-term
and termination by electron spin resonance and infrared spectrocopies. Macromolecules 18, 2092. Carlsson D. 3., Chan K. H., Garton A. and Wiles D. M. (1980) Photooxidative degradation of polypropylene and stabilization by hindered amines. Pure Appi. Chem. 52,
low-level exposure to radiation as observed in acrylic scintillator. Nuci. Instr. Meth. Phys. Res. A240, 262. Wallington T. 3., Maricq M. M., Ellermann T. and Nielsen 0. J. (1992) Novel method for the measurement of gas-phase peroxy radical absorption spectra. J. Phys. Chem. 96, 982. Wick K., Paul D., Schroder P., Stieber V. and Bicken B. (1991) Recovery and dose rate dependence of radiation damage in scintillators, wavelength shifters and light guides. Nuci. Instr. Meth. Phys. Res. 861, 472.
389. Clough R. (1988) Encylopedia of Polymer Science and Engineering, Vol. 13, 2nd edn, p. 667. Wiley, New York. Crowley 3. N., Simon F. 0., Burrows 3. P., Moortgat G. K., Jenkin M. E. and Cox R. A. (1991) The HO2 radical U.V.
0146.5724/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
EFFECT OF X IRRADIATION ON OPTICAL PROPERTIES OF TEFLON-AF 2 M. S. J.~ii~N,’D. R. Eiu.init’ and D. W. COOKE tDepartment of Physics, Memphis State University, Memphis TN 38152, U.S.A. and 2Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. (Received 9 June 1992; in revised form 14 August 1992) Abstract—Radiation effects in optical-grade amorphous fluoropolymer, Teflon-AF, is investigated by
UV—visible absorption and electron spin resonance (ESR) measurements. When irradiated with lowenergy (40 kVp) X-rays at room temperaturein air, Teflon-AF is found to develop a broad, structureless UV-absorption band in the wavelength interval 200—350 nm. While the UV absorption increases as a function of X-ray dose, with relative rates of approx 2 x l0~Gy~(I x l0-~Gy ‘)in Teflon-AF 1600 (l’eflon-AF 2400), its optical transparency for a given dose of 67.5 kGy, however, remains unaffected. Additional measurements conducted using electron spin resonance (ESR) technique reveal that the observed UV absorption is caused by the X-ray induced peroxy radical (P00). The results also suggest that the inclusion of dioxole monomer in the PTFE chain not only improves the optical clarity of Teflon-AF, as reported, but also increases its radiation tolerance. During a post-irradiation storage in air at RT for about 30 days the peroxy radical is observed to decay, with a concomitant decrease in UV absorption. A tentative model is proposed to explain the radiation damage and recovery mechanisms.
INTRODUC’IlON A new family of amorphous teflon (amorphous fluoropolymer) known as Teflon-AF (developed by Du Pont) is reported (Lowry et al., 1990) to have very high optical transmission property. In the visible—NIR regime the transmission is 95% or better, and in the UV (200—400 nm) it is about 80%. With this high optical transparency, in conjunction with its low surface energy, refractive index between 1.29 and 1.31, and resistance to severe environment (water absorption <0.01%), Teflon-AF shows a great potential for application in optics. For example, optical fibers, cladding for fiber bundles, lenses and a host of other optical components can be made out of this material. It can also be used for optical coatings. While preliminary studies (Lowry et a!., 1990) provide information about the mechanical and thermal stabilities, and the effects of y irradiation on the mechanical properties (elongation at break decreases by 12% and tensile t0Co) at break by 3 kPsi of Teflon-AF, for data no a dose areofavailable, 1 x l0~Gy to our of knowledge, on the radiation effects on its optical properties. This paper reports changes in optical absorption of Teflon-AF due to X irradiation at room temperature (RT) in air. The motivation for this study stems from our search for radiation tolerant fibers and cladding materials to be used in ionizing-radiation environments (detector systems in SSC, for example). Fibers and optical components will also be used in many environments where they may encounter radiation 481
doses sufficient to alter their transparency. Possible application sites are spacecraft, nuclear reactors, natural radioactive deposits, radioactive waste storage areas and the vicinity of nuclear weapons detonations. Studies on commercially available fibers and cladding materials show an appreciable degradation of optical transmission due to radiation damage. The growth and recovery of radiation-induced attenuations in 5i0 2, Ge-Si02 and Ge-P—Si02 were investigated by Friebele ci a!. (1984), who attributed this effect to the formation of color centers due to intrinsic and extrinsic defects. In recent studies, Jahan et a!. (199lb) observed formation of absorption bands due to y irradiation in acrylic light pipes and Wick ci a!. (1991) observed radiation damage and recovery in polymethyl-methyacrylate (PMMA)based light guides due to formation and decay, respectively, of free radicals. In this study free-radical growth and the concomitant change in UV—visible absorption were measured as a function of X-ray dose. Similar measurements were performed a function of storage (in air at RT)also time followingasa maximum dose of 67.5 kGy to monitor recovery of the materials. EXPERIMENTAL
Materials This study concentrated on two new generation teflon materials, Teflon-AF 1600 and Teflon-AF 2400. The Teflon-AF family is made of dioxole monomer and polytetrafluoroethylene (PTFE)
482
M. S.
JAHAN
(Lowry et a!., 1990). Shown below is its chemical structure. F
F
F
F
CF(
“èp3
~
2.5
et a!.
The difference between Teflon-AF 1600 and Teflon-AF 2400 is only in the relative amount of the dioxole monomer in the basic polymer chain. The numbers 1600 and 2400 refer to the glass transition temperatures of 160 and 240°C,respectively. Two pre-polished samples were provided by the Du Pont Corporation. Each of the samples was a compression moulded disk, approx 25 mm in diameter and 3.2 mm thick. Small pieces suitable for ESR and optical measurements were cut from these
(A) Teflon-AF 1600
2.0 ~ 2.4
1.5 Id
Q
a
Id
C
1.0
~0.8 0
I
310
.~
420
I
530
I
640
I
750
Wavelength (nm) 200
0
200
240
280
320
360
400
Wavelength (nm) 4.5
(B)
3.6 Teflon-AF 2400 2.7 I-
V
-
a I-
1.8
d
o
F.,
e
C
0.9
a
b
0 200
240
I 280
I 320
360
400
Wavelength (nm) Fig. I. UV-absorption spectra of X irradiated Teflon-AF 1600 (A) and Teflon-AF 2400 (8). Cumulative X-ray doses are 0.0 (a), 7.5 (b), 15.0 (c), 22.5 (d), 45.0 (e) and 67.5 kGy (f). The inset shows the UV—visible absorbance of the Teflon-AF 1600 following 67.5 kGy of X-ray dose.
Effect of X irradiation on optical properties of Teflon-AF
483
(A) ESR Teflon-AF 1600
25 Gauss
e
(B) Teflon-AF 2400
,
a
___
~
__.) -
—
b
,~
—
-.
C
d
25 Gauss e
Fig. 2. ESR spectra of X irradiated Teflon-AF 1600 (A) and Teflon-AF 2400 (B). Cumulative X-ray exposures are 0.0 (a), 7.5 (b), 15.0 (c), 22.5 (d), 45.0 (e) and 67.5 kGy (f). g~and g 1 are the parallel and perpendicular components, respectively, of the electronic splitting factor.
—
484
M. S.
JAHAN ci a!.
25-mm samples and were used without additional
ci a!., 1992) and hydrogen peroxy (HO
polishing.
ci a!., 1991) radicals, and in superoxide (O~peroxy) defect in topaz (A12F2(Si04)) (Priest ci a!., 1991).
Methods Samples were X-irradiated at room temperature in air employing an X-ray machine with a copper target and operating at 40 kVp and 20 mA. Dose rate at the sample site was 1.5 kGy/min. Maximum dose given to a sample was 67.5 kGy. UV—visible absorbance measurements were conducted using a UV—visible—NIR spectrophotometer (CARY 2390) at wavelengths ranging from 200 to 750 nm. Absorption spectra of a single sample were recorded following a cumulative dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. For ESR measurements a X-band spectrometer, whose cavity operates in TE102 mode with 50 kHz modulation frequency of the magnetic field, was employed. The spectrometer calibration and sensitivity tests, and the determination of free radical concentration in a sample were accomplished by using a ruby standard (NIST). A low microwave power was maintained in the cavity so as to avoid saturation of the ESR signals. Like absorbance measurements, ESR measurements were also conducted on a single sample following each cumulative X-ray dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. The total time a sample was exposed to air between two successive irradiations was, including the time for a measurement (ESR or optical absorption), approx 10 mm. Following a dose of 67.5 kGy the samples were stored in air at RT. Their recovery was then monitored by recording absorbance and ESR data at a regular interval of time ranging from 0 to 30 days.
From radiation damage study of PMMA-based light guides and wavelength shifters, and from a review of similar works by others, Wick ci a!. (1991) noted that the peroxy radicals absorb in the UV but not in the visible. An excellent illustration of this phenomenon is found in the present work. The ESR spectra, shown in Fig. 2, bear the characteristic features of a peroxy radical with anisotropic electronic splitting factors g11 = 2.0399 and g1 = 2.0175 (AH = 46G). A correlation between the peroxy radical concentration and the UV absorption is shown in Fig. 3. It is found that for a given X-ray dose the relative concentration (N16/N24)~,,of peroxy radicals (N16 = concentration of peroxy radicals in Teflon-AF 1600 and N24 = concentration of peroxy radicals in Teflon-AF 2400) is found to be 2.5. The relative absorbance (A16/A24)225 nm, (A16 = absorbance in Teflon-AF 1600 and A24 = absorbance in Teflon-Af 2400, both measured at 225 nm) obtained from the UV-absorption data (Fig. 3) is, however, found to be 2. This difference might be attributable to the methods used to compute them. Recall that radical concentration is obtained from a double integration of the entire ESR spectrum, while the relative absorbance (A16/A24)225 nm is computed at a particular wavelength (225 nm) of the broad UV band. Within experimental uncertainties, therefore, they can be considered approximately equal, (N16/N24) (A16/A24)225 nm. From this correlation one can condude that for a given X-ray dose, about twice as many peroxy radicals are formed in Teflon-AF 1600 than in Teflon-AF 2400, and that the peroxy radicals
RESULTS AND DISCUSSION
thus formed cause UV absorption. Peroxy radicals
Shown in Fig. 1 (A and B) are the absorption spectra of Teflon-AF 1600 and Teflon-AF 2400 recorded following cumulative X-ray dose of 0, 7.5, 15.0, 22.5, 45.0 and 67.5 kGy. Because there is no observable change in the absorbance due to X irradiation at wavelength 2 > 350 nm [see inset, Fig. 1(A)], only the UV (200—400 nm) spectral region is shown in the figures. The concomitant ESR spectra of the samples are shown in Fig. 2 (A and B). The area under an ESR absorption curve, which measures the relative concentration of free radical content in a sample, was obtained by integrating twice the recorded first derivative spectrum and plotting it as a function of X-ray dose (see Fig. 3). Also plotted in Fig. 3 are the absorbance (at an arbitrarily chosen 2 = 225 nm) as a function of X-ray dose It is clear from Fig. 1 (A and B) that Teflon-AF 1600 and Teflon-AF 2400, which differ only in relative absorbance, show similar UV absorption behavior. Their absorption bands spread continuously from 200 to about 350 tim. Similar UV absorptions were observed in methylperoxy (CH302) (Wallington
2) (Crowley
1.5
-
1 0
—
(a)
-
Teflon-AF ~
—
2
-
i
—
0 1.0 ~
•
i o.~ — ~ ~ .~.
0.8
—
0.6
—
,‘
I
I
I
(b)
-
Teflon-AF 2400
—
0.4 02 .
0
-
0.6 0.4
—
0.2
—
—
_•~4 20
40
60
80
X-ray dose (kGy) Fig. 3. Increase in UV-absorbance and integrated ESR intensity are plotted as a function of X-ray dose. (a) Teflon-AF 1600 and (b) Teflon-AF 2400.
Effect of X irradiation on optical properties of Teflon-AF
have been investigated in a large number of polymers, for example, polypropylene (PP) (Becker ci a!., 1988; Carlsson ci a!., 1985, 1980), polytetrafluoroethylene (PTFE) (Ranby and Rabek, 1975) and polyethylene (PE) (Igarashi, 1983; Jahan et a!., 1991a). The mechanisms of formation of peroxy radicals in Teflon-AF can be outlined as follows. Radiation-induced polymer free radicals (P) react with oxygen molecules (02), trapped in or diffused (from outside) into the matrix, to produce peroxy radicals. Xra
~ P
0~
P00
Initiation
(1)
Propagation
(2)
Recall that X irradiation of Teflon-AF and the subsequent ESR and UV—visible absorption measurements were conducted in air, thus making oxygen easily available to previously formed polymer radicals. Furthermore, oxygen, because of its high diffusional mobility and high reactivity with radicals, traps radicals efficiently to produce peroxy radicals, as shown by reaction (2), and promote oxidative reactions. This is consistent with the numerous reports (Sirois and Wigmans, 1985; Gillen and Clough, 1985; Moore and Choi, 1991) on radiationinduced free radicals in polymers and their reactions
1.0
485
(a)
-
Teflon-AF 1600
~
0.8 0.6
-\
0.4
-
0.2
-
1.0
-
UV-absorbance
ESR
~..
0.8 ~ 0.6
~
0.4 0 2
I b
I
\
I
I
‘~•i
I
Teflon-AF 2400
-\ ~
UV-absorbance
-
ESR 0
!—4---—-4-~_~.J_~
~
I
5
30
35
10
15
20
25
Storage time (day) Fig. 4. Fractional changes in UV-absorbance and integrated ESR intensity as a function ofpost-irradiation storage time in air at RT. (a) Teflon-AF 1600 and (b) Teflon-AF 2400.
the radicals in these two samples can be due to initial radical concentration difference [(N 16/N24)e,,= 2.5]. with 02 during and after irradiation (for a review, Based on our computation of the relative concensee Clough, 1988). tration of peroxy radicals [(Nj6/N24)~~r = 2.5] and the A peroxy radical can react with another neighbor- fact that the concentration of dioxole monomer in ing peroxy radical to form a neutral species and/or Teflon-AF 1600 is less than that in Teflon-AF 2400, it can combine with a hydrogen atom by pulling it (Lowry et a!., 1990) we propose TFE (tetrafluorofrom environmental H20 (note, there is no hydrogen ethylene) in the main copolymer chain as the most in the molecular structure of Teflon-AF) and form a probable site for formation of the peroxy radical hydroperoxide. This is shown by reactions (3) and (4). (P0(Y). Similar peroxy radicals were observed in 2P0(Y —* P0 P +0 (3’~ PTFE by Moriuchi et a!. (1970), in PE (polyethylene) 2 2 / by Igarashi (1983), and in ultra-high molecular P00
~
P0~H
(4)
A post-Irradiation storage of Teflon-AF in air ~ therefore, lead to a simultaneous decrease of the ESR and UV-absorption intensities of the peroxy radicals as shown in Fig. 4. It is apparent from the figure that, in the given condition (RT in air), the peroxy radicals decay at a faster rate for the first 5 days, then follow a slow-decay pathway. Without additional data the decay mechanisms cannot be elucidated; however, one can speculate that, initially, either both reactions (3) and (4) can take part in the decay process or that the hydrogen reaches the near-surface radicals faster by reaction (4) leading to a fast decay of the P00~radicals. Following this, P00 radicals will be sparsely populated in the matrix and subsequent decay will be dominated by reaction (4). In this reaction hydrogen (H20 molecules) must travel a long distance to react with P00 radicals, leading to a slow decay process. Using ESR data we find the relative magnitude of the fast (kf) and slow (k,) decay rates, (k~/k~)1640 and (kf/k,)24 20, in Teflon-AF 1600 and Teflon-AF 2400, respectively. This difference (a factor of 2) in the decay rates of
weight PE by Jahan ci a!. (1991a). Measurements are in progress to ascertain whether P00~is a chain or -~ d~ an enu YP~ra ica. CONCLUSIONS
The optical transparency of optical-fiber grade amorphous teflon, Teflon-AF, is found to be unaffected within a given dose of low energy (40 kVp) X irradiation. This material, however, develops a UV-absorption band in the wavelength interval 200—350 nm when it is X-irradiated at RT in air. This UV band is broad and structureless and shows a continuous decrease in absorbance as wavelength increases from 200 to 350 nm. By comparing this result with previously published works, and by correlating it with ESR data, we conclude that the observed UV-absorption is caused by the X-ray induced peroxy radical (P0(Y). Furthermore, by comparing ESR results of Teflon-AF 1600 (containing more PTFE than dioxole monomer in the copolymer chain) with those of Teflon-AF 2400 (containing less PTFE than dioxole monomer in the copolymer
486
M. S.
JAHAN
chain) we conclude that the peroxy radical forms in the TFE segment of the copolymer chain. it is not clear, however, whether this radical is a chain or end (scission) type. Measurements are in progress to clarify this assignment. Without additional measurements post-irradiation decay mechanisms of the peroxy radicals, with concomitant decrease in the UV absorption, cannot be elucidated; however, we present a tentative model that explains the experimental results. Initially, when radical concentration is high, a peroxy radical cornbines with another neighboring peroxy radical and/or a hydrogen pulled from the environmental H 20. This reaction gives rise to a fast decay process, yielding sparsely populated radicals in the polymer matrix. Subsequent decay will be slow because of the reduction in the radical—radical recornbination and, the long distance required for hydrogen to travel before it can find a peroxy radical with which to combine, Acknowledgements—This research is supported in part by the Texas National Research Laboratory Commission through SAHEP Initiative. We are thankful to Du Pont and to R. L. Clough of Sandia National Laboratory for providing the materials used in this study.
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